Front corners are high-stress points that crack. This one has already been welded back together (welded from the back, so the weld isn’t very visible.)

Back side also glued with polyurethane glue. Almost nothing sticks to polypropylene, so the glue might peel once it’s back in use… We’ll see if it’s good enough.

The plastic is scored with a knife to make a rougher surface so the glue might hold better.

Next step: Reinforce the weak spot using some fiberglass window screen soaked in epoxy. Or maybe epoxy-polyurethane mix. Something mostly rigid, but not so hard that it cracks when flexed slightly, hopefully.

Fibreglass window screen partially glued with a bit of epoxy.

Window screen covered with epoxy.

Update

Two months later, the crack hasn’t spread. Cosmetically, the crack is visible. Structurally, the repair seems good enough.